In the field of satellite communications, a growing need has arisen for earth station antennas that are inexpensive to construct and easy to operate in order to change the orientation of the reflector to aim at any one of a number of geosynchronous satellites. In order to permit changes in its orientation, an antenna reflector must pivot or rotate about one or more axes, depending on the type of mounting utilized.
One typical antenna mounting structure is the elevation-over-azimuth type, in which bearing structures must be provided for independent rotation about the vertical or azimuth axis and about the horizontal or elevation axis. The elevation axis assembly should allow the reflector to be pointed from slightly below the horizon to high above the horizon. The azimuth bearing assembly has utility proportional to the degree of rotation permitted; optimal utility is realized if the reflector can rotate 360.degree. about the azimuth axis. Some typical azimuth bearings provide such flexibility, and some do not. For example, in a spindle-type azimuth mounting, the reflector is attached to a vertical rod rotatably mounted on bearings extending from a support structure. Rotation through 360.degree. is not possible, because the reflector cannot swing past the mounting structure in typical installations. Also, the support structure must be relatively massive in order to provide stability.
Rotation through 360.degree. and stability has been provided by another typical azimuth bearing system, in which the antenna reflector is mounted on a large circular bearing, such as a roller bearing 2-10 feet in diameter, and the bearing is carried in a circular race. Stability is gained by increasing the diameter of the circular bearing, which also steeply increases the cost of this type of azimuth bearing system.
The polar reflector mounting structure is a widely used alternative to the elevation-over-azimuth system. As a result of necessary positioning of the orbits of geosynchronous satellites on the equitorial plane, an antenna reflector can move from one satellite to another by rotation about a single axis slanted with respect to the horizon and oriented in the North-South plane. The azimuth position of such an antenna must be initially fixed to place the polar axis in the North-South plane, and therefore it is best to provide an azimuth bearing assembly to facilitate fine adjustment of the azimuth position after the base of the antenna mounting structure is secured to a foundation. An elevation assembly is required to permit additional precise adjustment of the slant of the polar axis.
In addition to permitting rotation of the antenna reflector, bearing assemblies associated with antenna mounting structures must have means for locking the position of the antenna about the various axes. The pointing accuracy of an antenna aimed at a satellite must be within about 0.1.degree.-0.25.degree.. Thus, convenient and accurate positioning of an antenna requires that the bearing assemblies be lockable without motion of the antenna during the locking procedure. As a result of the various requirements for an acceptable antenna mounting structure, such structures have generally been constructed of heavy duty materials, often including expensive precision bearings. As the demand for satellite antennas has increased, the need for an inexpensive mounting structure providing the required precision adjustments has become more acute.